A fuel cell has a cathode, an electrolyte, and an anode. An oxidation agent, for example air, is fed to the cathode, and a fuel, for example hydrogen, is fed to the anode.
Various types of fuel cells are known, such as the SOFC fuel cell from the published patent DE 44 30 958 C1, as well as the PEM fuel cell from the published patent DE 195 31 852 C1.
The SOFC fuel cell is also called a high-temperature fuel cell, since its operating temperature can reach up to 1000° C. On the cathode of a high-temperature fuel cell, oxygen ions are formed in the presence of the oxidation agent. The oxygen ions are diffused through the electrolyte and recombine into water on the anode side with the hydrogen deriving from the fuel. During recombination, electrons are released, producing electric energy.
Typically, in order to achieve a high electric output, several fuel cells are electrically and mechanically connected to each other by means of connecting elements, also known as interconnectors. These interconnectors form fuel cells that are stacked on top of each other and electrically connected in series. This arrangement is called a fuel-cell stack. The fuel-cell stacks are comprised of the interconnectors and the electrode-electrolyte units.
In addition to their electrical and mechanical properties, interconnectors regularly also have gas distribution structures. This is accomplished by means of ridges and grooves (DE 44 10 711 C1 [U.S. Pat. No. 5,733,682]). Gas-distribution structures have the effect that the operating agents are evenly distributed in the electrode spaces (spaces where the electrodes are located).
The disadvantage with fuel cells and fuel-cell stacks is that the following problems can occur:
metallic interconnectors having a high aluminum content form Al2O3 cover layers which disadvantageously act like an electrical insulator.
during a cyclical temperature load, heat tensions generally occur in connection with the relative movements of the individual components; these are a result of the different expansion behavior and/or the different expansion coefficients of the materials used during the operation.
In this regard, the state of the art does not yet provide for sufficient compatibility between the comparatively high expansion coefficients of the metallic interconnector and the current electrode materials, for example, whose expansion coefficients are comparatively small. On the one hand, heat tensions can occur between electrodes and interconnectors and can cause destruction within the fuel cell. On the other hand, this can also relate to the glass solders that are frequently used in fuel cells and are supposed to ensure the impermeability of the fuel cells. During the joining process, the fuel-cell stack is heated to approximately 700-900° C. and pressed together at 1-5 kN. This causes the glass solder to soften, so that the joining pressure not only causes gaps between the cells, the interconnectors, and the housing to be sealed, but at the same time results in a contact pressure to create an electrical contact between the cells and the interconnectors.
A disadvantage in this arrangement is that the glass solder crystallizes after only a few hours of operation and becomes brittle and hard. Its elastic properties are lost. As a result, the contact pressure acting on the outside of the stack is distributed over the outer sealing force and the inner contact force in an irregular and uncontrolled fashion. Moreover, when the fuel-cell stack is operated at 700-900° C. for an extended period of time, creeping occurs in the various layers of stack materials, and particularly shrinking in the initially unsintered cathode contact layer. This makes it impossible to maintain a reliable contact force between the cells and interconnectors, and the electrical contact is lost. The fuel cell is no longer functional.